The present invention relates to reduction of iron bearing materials such as iron ore to metallic iron nodules (known as “NRI”).
Metallic iron has been produced by reducing iron oxide such as iron ores, iron pellets, and other iron sources. Various such methods have been proposed so far for directly producing metallic iron from iron ores or iron oxide pellets by using with reducing agents such as coal or other carbonaceous material. Such fusion reduction processes generally involve the following processing steps: feed preparation, drying, preheating, reduction, fusion/melting, cooling, product discharge, and metallic iron/slag product separation. These processes result in direct reduction of iron bearing material to metallic iron nodules (NRI) and slag. Metallic iron nodules produced by these direct reduction processes are characterized by near total reduction, approaching 100% metal (e.g., about 96% or more metallic Fe). Percents (%) herein are percents by weight unless otherwise stated.
Unlike conventional direct reduced iron (DRI) product, the metallic iron nodule (NRI) product has little or no gangue and little or no porosity. NRI is essentially metallic iron product desirable for many applications, such as use in place of scrap in steelmaking by electric arc furnaces. Such metallic iron nodules may be made by processing beneficiated taconite iron ore, which may contain 30% oxygen and 5% gangue. In addition to advantages of the NRI product, there is less bulk to transport than with beneficiated taconite pellets or DRI, as well as a lower rate of oxidation and a lower porosity than DRI. In addition, generally, such metallic iron nodules are just as easy to handle as taconite pellets and DRI. Moreover, NRI is a more efficient and effective substitute for scrap in steel making by electric arc furnace (EAF) without extending heat times and increasing energy cost in making steel.
Various types of hearth furnaces have been described and used for direct reduction of metallic iron nodules (NRI). One type of hearth furnace used to make NRI is a rotary hearth furnace (RHF). The rotary hearth furnace is partitioned annularly into a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the supply location and the discharge location of the furnace. An annular hearth is supported rotationally in the furnace to move from zone to zone carrying reducible material the successive zones. In operation, the reducible material comprises a mixture of iron ore or other iron oxide source and reducing material such as carbonaceous material, which is charged onto the annular hearth and initially subject to the drying/preheat zone. After drying and preheating, the reducible material is moved by the rotating annular hearth to the reduction zone where the iron ore is reduced in the presence of the reducing material, and then to the fusion zone where the reduced reducible material is fused into metallic iron nodules, using one or more heating sources (e.g., natural gas burners). The reduced and fused NRI product, after completion of the reduction process, is cooled on the moving annular hearth in the cooling zone to prevent reoxidation and facilitate discharge from the furnace. Another type of furnace used for making NRI is the linear hearth furnace such as described in U.S. Pat. No. 7,413,592, where similarly prepared mixtures of reducible material are moved on moving hearth sections or cars through a drying/preheating zone, a reduction zone, a fusion zone, and a cooling zone, between the charging end and discharging end of a linear furnace while being heated above the melting point of iron.
A problem in the production of NRI has been the decarburization and reoxidation of iron during and after the fusion of reduced iron reducible material into metallic iron. Oxygen present in the fusion zone may combine with carbon and iron on the surface of the metallic iron nodules forming CO and FeO. Various solutions have been presented to overcome this problem such described in Moon and Sahajwalla, “Investigation into the Role of the Boudouard Reaction in Self-Reducing Iron Oxide and Carbon Briquettes,” Metallurgical and Materials Transactions, Vol. 37B at 215 (April 2006).
We have found that providing a coarse carbonaceous overlayer over the reducible material results in higher efficiency reduction of the reducible material to metallic iron nodules, and a markedly lower percent of the sulfur in the metallic iron nodules. Additionally, we have found providing the coarse carbonaceous overlayer reduced the formation of micro-nuggets (small metallic iron nodules having a size between about 20 mesh and about 3 mesh). However, adding the coarse overlayer prior to the preheat zone means the coarse overlayer is heated and partially consumed in the drying zone and reduction zone. We have also found a method of producing metallic iron nodules with more effective and efficient use of the coarse overlayer by introduction into the furnace near or in the fusion zone avoiding reduction and consumption of the coarse overlayer material in the drying and reduction zone. We have also found that this method results in more efficient and effective fusion of the metallic iron nodules with less fuel consumption and without loss of iron units to slag and unreduced reducible material.
A method for producing metallic iron nodules is disclosed comprising the steps of:                providing a hearth material layer comprising at least carbonaceous material on a refractory hearth in a traveling hearth furnace,        providing at least one layer of reducible material comprising at least reducing material and reducible iron bearing material arranged in a plurality of discrete compacts over at least a portion of the hearth material layer,        heating the reducible material in a drying/heating atmosphere and then in a reducing atmosphere to at least partially reduce the reducible iron bearing material,        assembling a shielding entry system to introduce coarse carbonaceous material greater than 6 mesh in particle size into the furnace atmosphere in at least one location such that the temperature of the furnace atmosphere adjacent the at least partially reduced reducible iron bearing material is between about 2200 and 2650° F. (1200 to 1455° C.), the shielding entry system adapted to inhibit emission of infrared radiation from the furnace atmosphere and seal the furnace atmosphere from exterior atmosphere while introducing coarse carbonaceous material greater than 6 mesh into the furnace to be distributed over the at least partially reduced reducible iron bearing material,        introducing a coarse carbonaceous material of greater than 6 mesh in particle size through the shielding entry system into the upper portion of the furnace, and        heating the at least partially reduced reducible iron bearing material in a fusion atmosphere to form from the at least partially reduced reducible iron bearing material by fusing one or more metallic iron nodules and slag with the coarse carbonaceous material to assist in fusion and inhibit reoxidation of the reduced material during fusion.        
The compacts may be preformed in briquettes, balls, pellets other form or formed in situ as described in U.S. Patent Publication 2006/0150774 (now allowed), with or without a binder. The coarse carbonaceous material may be selected from the group consisting of anthracite coal, bituminous coal, PRB coal, coke, char, and mixtures of two or more thereof. The coarse carbonaceous material introduced into the furnace atmosphere may have a particle size between 6 mesh and ½ inch or between 4 mesh and ½ inch.
The carbonaceous material may be introduced at a temperature between about 2300° F. and 2500° F. (1260 and 1370° C.). The coarse material may be introduced in the upper part of the furnace and distributed over the reducible material, or guided to within 6 inches or less and distributed over the heated reducible material. The carbonaceous material may be introduced as a layer over the at least partially reduced reducible iron bearing material. Alternatively or in addition, the coarse carbonaceous material may introduced at a rate corresponding to between about 0.25 lb./ft2 (1.22 kg/m2) and about 1.25 lb./ft2 (6.10 kg/m2), and/or coverage of the coarse carbonaceous material is between about 0.25 lb./ft2 (1.22 kg/m2) and about 1.25 lb./ft2 (6.10 kg/m2). In particular, the coarse carbonaceous material may be PRB coal introduced at a rate corresponding to between about 0.5 lb./ft2 (2.44 kg/m2) and about 1.25 lb./ft2 (6.10 kg/m2).
The shielding entry system may be assembled with at least one stepped labyrinth no more than 6 inches wide in the direction of movement of the at least partially reduced reducible iron bearing material through the furnace such that the emission of infrared radiation from the furnace atmosphere is inhibited. Alternatively or in addition, the shielding entry system may be a pellet ladder no more than 2 inches wide in the direction of movement of the at least partially reduced the reducible iron bearing material through the furnace to inhibit emission of infrared radiation from the furnace atmosphere. The shielding entry system may also seal the furnace atmosphere from the exterior atmosphere (with some leakage tolerated according to commercial efficiency).
The assembled shielding entry system may be slanted to delivery of the coarse carbonaceous material over the at least partially reduced reducible iron bearing material in the direction of flow of atmosphere through the furnace. Alternative or in addition, the assembled shielding entry system is slanted to delivery of coarse carbonaceous material over the at least partially reduced reducible iron bearing material in the direction thereof through the furnace.
Further, the shielding entry system may be assembled to permit introduction coarse carbonaceous material greater than 6 or 4 mesh in particle size at multiple locations where furnace atmosphere is between about 2200 and 2650° F. (1200 and 1450° C.) at each location, such that the shielding entry system inhibits emission of infrared radiation from the furnace atmosphere at each such location, and the coarse carbonaceous material of greater than 6 or 4 mesh in particle size is introduced into the furnace atmosphere through the shielding entry system into the upper portion of the furnace at more than one location along the furnace.
In any event, the shielding entry system may comprise sealing portions of the shielding entry system against substantial egress of furnace atmosphere. The assembled shielding entry system may comprise a feed mechanism adapted to enable controlled distribution of the coarse carbonaceous material across the width of the at least partially reduced the reducible iron bearing material.
The method may further include the step of introducing a carrier gas in the shielding entry system to inhibit oxidation of the coarse carbonaceous material therein. This addition step may further include elutriating in the shielding entry system carbonaceous material with a gas selected from the group consisting of nitrogen, carbon dioxide, carbon monoxide, recycled furnace gas, and mixtures thereof. An amount of oxygen may also be introduced with the carrier gas less than the stoichiometric amount for oxidation of the coarse carbonaceous material.
A method of producing metallic iron nodules is disclosed comprising the steps of:
providing a hearth material layer comprising at least carbonaceous material on a refractory hearth in a traveling hearth furnace,
providing at least one layer of reducible material comprising at least reducing material and reducible iron bearing material arranged in a plurality of discrete compacts over at least a portion of the hearth material layer,
heating the reducible material in a drying/heating atmosphere, then in a reducing atmosphere to at least partially reduce the reducible iron bearing material, and then in a fusion atmosphere to form by fusion one or more metallic iron nodules and slag,
assembling a shielding entry system to introduce into the furnace atmosphere adjacent introduction of the reducible material at a temperature between about 2200 and 2650° F. (1200 and 1450° C.) while inhibiting emission of infrared radiation from the furnace atmosphere, shielding entry system adapted to seal the furnace atmosphere from the exterior atmosphere and guide coarse carbonaceous material having a particle size greater than 6 mesh or 4 mesh to within about six (6) inches of the heated reducible material and distribute the coarse carbonaceous material over the heated reducible material in the furnace, and
introducing a coarse carbonaceous material having particle size greater than 6 mesh or 4 mesh through the shielding entry system over the heated reducible material to assist in fusion and inhibit reoxidation of the reduced material during fusion in forming metallic iron nodules.
The coarse carbonaceous material may be selected from the group consisting of anthracite coal, bituminous coal, sub-bituminous coal, PRB coal, coke, char, and mixtures of two or more thereof. The coarse carbonaceous material introduced into the furnace atmosphere may have a particle size between 6 mesh and ½ inch or between 4 mesh and ½ inch. For example, the coarse carbonaceous material may be coal and introduced as a layer over the heated reducible material. In some embodiments of the method, introduction and/or coverage of the coarse carbonaceous material may be between about 0.25 lb./ft2 (1.22 kg/m2) and about 1.25 lb./ft2 (6.10 kg/m2). Alternatively, the introduction and/or coverage of the coarse carbonaceous material may be between about 0.25 lb./ft2 (1.22 kg/m2) and about 0.75 lb./ft2 (3.66 kg/m2). The overlayer may include coarse carbonaceous material containing volatiles in desired amount to enhance fusion and slag separation and provide additional thermal energy to the fusion zone.
In the method for producing metallic iron nodules, the step of assembling the shielding entry system may include the shielding entry system provided with a stepped labyrinth no more than 2 inches wide in the direction of movement of the heated reducible material through the furnace. More particularly, the shielding entry system may be a pellet ladder. In addition, the step of assembling a shielding entry system into the furnace may include assembling the shielding entry system adapted to guide the coarse carbonaceous material to within about three (3) inches of the heated reducible material. In alternative or in addition, the step of assembling the shielding entry system may include the shielding entry system being roof or wall lances, or both. The shielding entry system may be substantially vertical or alternatively, may be slanted to deliver the coarse carbonaceous material distributed over the at least partially reduced reducible iron bearing material in the direction of movement of the hearth and/or the furnace gases through the furnace.
The step of assembling a shielding entry system may involve sealing the fusion atmosphere from the exterior atmosphere with some leakage tolerated for commercial efficiency. In addition, the step of assembling a shielding entry system may include delivering downwardly an inert gas in the shielding entry system to inhibit oxidation of the carbonaceous material.
The shielding entry system may be adapted to providing a substantially uniform distribution of carbonaceous material across the width of the reducible material. The method for producing metallic iron nodules further may comprise elutriating carbonaceous material with a carrier gas selected from the group consisting of nitrogen, carbon dioxide, carbon monoxide, recycled furnace gas, and mixtures thereof. The gas may be selected such that oxygen in the gas is less than the stoichiometric amount needed for oxidation of the carbonaceous material.
The method of producing metallic iron may include providing a hearth material layer comprising a plurality of carbonaceous material layers on the moving refractory hearth traveling through hearth furnace, with the hearth material layer comprising a layer of undevolatized (e.g., fresh) coal and a layer of devolatilized carbonaceous material over the undevolatized coal. The undevolatized coal in the hearth material layer may be selected from the group consisting of anthracite coal, bituminous coal, sub-bituminous coal, and mixtures thereof. The devolatilized carbonaceous material in the hearth material layer may be char or coke. The devolatilized carbonaceous material in the hearth material layer may be carbonaceous material remains removed from the hearth at the exit end of the furnace (with or without the ash from NRI formation).
The steps of the method for producing metallic iron nodules may be performed in a linear hearth furnace. Alternatively, the steps of the method for producing metallic iron nodules are performed in a rotary hearth furnace.